Up Counters in PLC Ladder Logic

The Up Counters (CTU) is an instruction that counts false-to-true rung transitions in ladder logics.

Rung transitions can be caused by events occurring in the program (from internal logic or by external devices) such as parts traveling past a detector or actuating a limit switch.

Up Counter (CTU)

When rung conditions for a CTU instruction have made a false-to-true transition, the accumulated value is incremented by one count, provided that the rung containing the CTU instruction is evaluated between these transitions.

The ability of the counter to detect false–to–true transitions depends on the speed (frequency) of the incoming signal.

The accumulated value is retained when the rung conditions again become false.

The accumulated count is retained until cleared by a reset (RES) instruction.

Up Counter Example

Accumulated count is reset only by the (RES) instruction.

The counter will increment the accumulator value even after the preset is reached.

Counter Status Bits


/CU: count up
/CD: count down
/DN: counter done
/OV: counter overflow
/UN: counter underflow

Using Status Bits (CTU)

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